Radiotherapeutic apparatus and techniques have now developed to the point where it is likely that some form of tracking or gating system to reduce the effects of tumor motion (primarily due to breathing) will soon be feasible. Such techniques offer significant advantages in terms of the treatment of tumors in the chest and lung area. They require, however, a source of information as to the current location of the tumor, during treatment, in real time. Full 3-D imaging of the tumor requires significant computing time, which introduces an unacceptable lag between the image acquisition and the treatment.
Most existing respiration correlation systems use some form of surrogate monitoring system which has a fast response time, in conjunction with a correlation to the internal tumor position. In other words, rather than try to achieve the difficult task of monitoring the actual position of the tumor, either a feature in the patient is identified, or a marker is provided, whose position can more easily be tracked and is correlated with that of the tumor.
One such method is disclosed in our earlier PCT application WO 2010/066265. In that application, many breathing phases are sampled, and therefore the correlation between the surrogate position and the tumor position is an average with a high statistical confidence. However, once imaging of the patient has stopped, the relationship between the surrogate and the target position is assumed from the earlier measurements. Any change in the surrogate-target relationship over time will cause inaccuracy in the targeting of the therapeutic radiation beam.
Other methods use only a few breathing cycles to determine the correlation between the surrogate and the target region. This enables the correlation to be determined rapidly, and new correlations can be determined during treatment. However, as they are based on a small number of breathing cycles, such methods are susceptible to the sampled breathing cycles not being representative of the ongoing surrogate correlation.